Semiconductor diode lasers have been used extensively as transmitters for fiber-optic communications. In one common and low-cost implementation, edges of two-opposing end facets of a semiconductor gain media are cleaved and/or polished to form resonant-reflective surfaces and to provide feedback for laser operation. This type of laser, commonly called a Fabry-Perot (FP) laser, typically emits light in multiple longitudinal modes and has large output line-widths.
In Dense-Wavelength-Division Multiplexing (DWDM) fiber-optic communication technology, many closely-spaced wavelengths or channels are transmitted simultaneously along a single fiber or fiber bundle. Typical spacing of channels in DWDM systems can range from 5 nanometers (nm) to as little as 0.4 nm (for 50 GHz International Telecommunication Union (ITU) standard channel spacing) between channels. Closer channel spacings are envisioned. This creates the need for a laser with a much narrower output line-width (i.e., frequency range) than a typical FP laser. To implement effective DWDM systems, stable and accurate transmitters of selectively predetermined wavelengths are needed for individual channels. In addition, stable and accurate wavelength-selective receivers are needed to selectively remove or receive the individual channel wavelengths to minimize crosstalk from other channels. For a DWDM system to operate efficiently, the transmitter and receiver for a given channel should be tuned to the same wavelength within a given tolerance.
There are at least two reasons why it would be advantageous to replace fixed wavelength lasers by tunable lasers in DWDM applications. The first reason has to do with inventory requirements. A typical DWDM system with 50 GHz-channel spacing uses one fixed-wavelength transmitter for each of 80 or more different wavelengths. Consequently, 80 individual fixed wavelength lasers have to be kept on hand to fabricate new systems and as spares to guard against the possibility of laser failure. However, all 80 of these fixed wavelength spare lasers could be replaced by a single tunable laser. The availability of tunable lasers would represent a very significant savings in inventory costs. The second advantage associated with tunable lasers is that their use would allow the configuration of a communication network to be changed dynamically.
A tunable external cavity laser (ECL) is a strong candidate for such applications. A tunable ECL is a laser device that can be configured to generate light of a particular wavelength within a tuning range. This type of device is described, for example, in “Widely Tunable External Cavity Diode Lasers,” Day et al., SPIE, Vol. 2378, P. 35-41. In the diode laser devices described by Day et al., an anti-reflective layer is placed on one facet of a semiconductor diode that provides an optical gain medium. A collimating lens captures the emitted light, and a diffraction grating, acting in part as an external cavity reflector, is used to select or tune the wavelength λ, of the laser. Lasing action occurs, generally, provided that the grating selects a wavelength within the diode's spectral gain region and where a substantial portion of the light is returned to the optical gain medium. Tuning action occurs when a properly positioned diffraction grating is controllably rotated about a fixed axis. Tuning may also involve adjusting the length of the external cavity between the back facet of the diode and the tuning mirror.
As illustrated in FIG. 1, ECL 10 includes a semiconductor light-emitting diode 11 having a reflective rear facet 18, a collimating lens 12, a diffraction grating 14, and a tuning mirror 16. The diode 11 serves as an optical gain medium. The diffracting surface 15 of the diffraction grating 14 and the reflecting surface 17 of the tuning mirror 16 are arranged such that tangent lines extended along surfaces 15, 17 intersect at pivot point 30. Neglecting the refractive indices of the optical elements in the optical cavity 13, the diode 11 is placed such that a tangent line extended along its rear facet 18 intersects with the other tangent lines at pivot point 30. The reflective rear facet 18 of the diode 11 and the reflecting surface 17 define the ends of external cavity 13. When the tuning mirror 16 is attached to a member (not shown for simplicity of illustration) that controllably rotates about pivot point 30, the wavelength of the ECL 10 is continuously tunable.
In operation, the diode 11 emits light forming a first beam portion 20a that is incident on the diffracting surface 15 of the diffraction grating at an angle of incidence, φINC. The first beam portion 20a is diffracted at an angle, θ, from the diffracting surface 15 of the diffraction grating 14. The diffraction angle, θ, is defined by the angle between the tuning mirror 16 and the diffraction grating 14. The diffracted light forms a second beam portion 20b directed at the reflecting surface 17 of the tuning mirror 16. When, as here, the second beam portion 20b impinges orthogonally on the reflecting surface 17 of the tuning mirror 16, a substantial portion of the light is reflected back toward the diode 11 in the reverse direction of the second beam portion 20b and the first beam portion 20a. 
The diffraction grating 14 and tuning mirror 16 collectively constitute a wavelength filter. The tuning mirror 16 defines one end of the external cavity 13 of ECL 10 and is mounted on a pivot. The angle of rotation of the tuning mirror 16 about the pivot selects a specific diffracted wavelength and adjusts the optical path length of the external cavity 13 to be an integral multiple of the selected wavelength. Light propagates between the optical gain medium of the diode 11, the diffraction grating 14, and the tuning mirror 16. For a given grating pitch, pg, and angle of incidence, φINC, of the light incident on the diffraction grating 14, the diffraction angle, θ, as selected by the rotation angle of the reflecting surface 17 of the tuning mirror 16 about pivot point 30, determines the operating wavelength, λ, through the following relationship:λ=pg[sin θ+sin φINC].
To provide continuous wavelength tuning without mode hops, lines tangential to the surfaces of the diffraction grating 14, the tuning mirror 16, and the reflective rear facet 18 should intersect at a common pivot point 30 as illustrated in FIG. 1. To tune the laser, the tuning mirror 16 is rotated in such a way that the tangent to its reflecting surface 17 always passes through pivot point 30. The general configuration shown in FIG. 1 is commonly called a Littman ECL or a Littman configuration.
A Littrow ECL (not shown) consists of a diode, a collimating lens, and a reflective diffraction grating. The diode, which serves as an optical gain medium, may be anti-reflection coated. However, it is possible to operate the system with standard diodes. The collimated light is coupled to the diffraction grating. The first order diffraction beam is directed back into the diode. The zeroth order diffraction beam is coupled out of the laser. One of the advantages of this laser design is that it is possible to achieve higher output power than with other types of laser systems.
The arrangement illustrated in the Littman ECL 10 is an easy way to visualize external laser cavity construction, but the arrangement only provides continuous wavelength tuning without mode hops in the case where the index of refraction is uniform throughout the external cavity 13. In reality, the external cavity of an ECL contains elements with different indices of refraction. For example, the refractive index of the optical gain medium is generally about 3.5 and the index of refraction of most collimating lenses is about 1.5. Consequently, to provide continuous wavelength tuning, the diode 11 and the collimating lens 12 are translated along the path of first beam portion 20a. With correct translation of the diode 11 and the above mentioned alignment of the other components (i.e., diffracting surface 15 of the diffraction grating 14, reflecting surface 17 of the tuning mirror 16, and pivot point 30), continuous wavelength tuning can be obtained. Similarly, in the Littrow configuration, correct translation of the optical gain medium, a collimating lens, and the reflective diffraction grating are required for continuous wavelength tuning.
The required mechanical alignment and translation tolerance of the optical components in an external cavity laser to achieve a continuous wavelength tunable configuration (e.g., in the Littman ECL 10 illustrated and described in FIG. 1, as well as in a Littrow configuration ECL) is on the order of micrometers. Implementing this level of accuracy in alignment is a severe challenge because of manufacturing tolerances, the capabilities of contemporary assembly equipment, and positional variability due to mounting adhesives. All these factors affect alignment of the optical components. An additional factor that contributes to the alignment challenge is that the length and refractive index of a particular diode optical gain medium are not known before fabrication of the assembly. This additional uncertainty may add tens of micrometers to the above-mentioned alignment errors.
One prior art approach to aligning the components in an ECL is to first assemble the components in an approximate arrangement. Next, a post-fabrication alignment is performed. The post-fabrication alignment is typically conducted by shifting the diode 11 along its optical axis (i.e., along the path defined by the first beam portion 20a) to adjust the physical length of the optical cavity until continuous tuning is achieved.
Generally, the diode 11 is shifted while monitoring the emitted output. At a particular diode position, the number of mode hops observed when the tuning mirror 16 is rotated about pivot 30 will be minimized and/or eliminated, thus establishing the optimum position for the diode 11. This post-fabrication alignment procedure uses a high-resolution translation mechanism. The translation mechanism is typically used only once in the lifetime of each ECL 10 to correctly position the diode 11 in the external cavity 13. The high-resolution translation mechanism adds significantly to the cost of the ECL 10. Alternatively, a reusable fixture can be used to correctly locate the diode 11 during assembly.
In light of pressures to reduce the manufacturing challenges associated with precise alignment, as well as competitive pressures to reduce the cost of tunable ECL devices, it can be appreciated that there is a need for systems and methods that address the above-described and/or other shortcomings of the prior art, while providing a manufacturable working device.